In processes for producing semiconductor devices and LCDs, a resist processing to a processing substrate is carried out by a technique called photolithography. This technique is carried out by a series of steps of applying a resist solution on, e.g., a semiconductor wafer (which will be hereinafter referred to as a wafer), to form a liquid film on the surface of the wafer, exposing the resist film by a photo mask, and thereafter, carrying out a developing processing. In such steps, twice heating processes called PAB (Post Applied Bake) and PEB (Post Exposure Bake) are carried out before and after the exposure processing. The heating processing in the PAB uses a system shown in, e.g., FIG. 8.
This system comprises a heating plate 14 including a heater 13 in a processing vessel 1 which comprises a lower portion 11 and a lid 12. On the heating plate 14, protrusions 15 are provided. When a semiconductor wafer (which will be hereinafter referred to as a wafer) being a substrate is held in a horizontal attitude to be mounted on the heating plate 14, a gap of a very small distance, e.g., 0.1, is formed between the wafer W and the surface of the heating plate 14 so as to prevent particles from adhering to the surface of the heating plate 14. The lid 12 is connected to a gas supply pipe (not shown) for supplying, e.g., a purge gas of a room temperature, into the processing vessel 1 and to an exhaust pipe (not shown) for exhausting from the processing vessel 1. These serve to form an air flow in the vicinity of the surface of the wafer W during a process to prevent a temperature distribution from being caused on the surface.
The control of the temperature of the wafer W in the above described system is carried out by detecting the temperature of the surface portion of the heating plate 14 by means of a temperature sensor 16, feeding back a detected signal to a control system 17, and controlling the power of the heater 13 so that the temperature of the surface portion of the heating plate 14 is held at a predetermined process temperature Various conditions during the process are constant until the end of the process from the start thereof. Specifically, the wafer W is heated to, e.g., 100° C., and this state is continued for 90 seconds to complete the processing.
By the way, the inventor has grasped that the line width of a resist pattern finally formed on the surface of the wafer W varies when processing conditions vary in the PAB if a certain resist (e.g., KrF-acetal) is used. Therefore, the PAB has some influence on the quality of the resist film. However, it has not been understood how to view the processing conditions to stabilize the quality of the resist film.
As described above, the object of the PAB is to heat (bake) the wafer W, on which the resist solution is applied, before exposure to volatilize a predetermined amount of a solvent contained in the resist solution. Therefore, the inventor has noticed the heating time to examine the correlation between the heating time and the resist film after the processing, and has noticed characteristics (Rmin) of the resist film to learn that there is a predetermined correlation between the characteristics of the resist film and the heating time. The Rmin shows a thickness h1 (see FIG. 9(a)) of a dissolved resist film when a developing solution is supplied to an unexposed resist film which has completely heated in the PAB, and means that, as the value of h1 is smaller, the resist film is more stable to the developing solution, so that the inplane uniformity of the quality of the film is higher to increase the inplane uniformity of the line width.
Then, as shown in a characteristic diagram of FIG. 9(b), the value of Rmin is lowest at a heating time of t0, and a V shape is drawn about the lowest value. The reason why the value of Rmin thus varies is considered that the resist film is not substantially solidified (not stabilized) until to since the heating time is short. The Rmin after t0 is considered as follows. That is, since the decrease of the resist film during baking is mainly caused by the volatilization of the solvent, the total amount of a polymer forming resist components contained in the resist solution does not vary as shown in, e.g., FIG. 10(a), and the proportion of the polymer in the whole resist film gradually increases. On the other hand, heating conditions are constant as described above, the proportion of thermal energy received by the polymer increases as the thickness of the film decreases as shown in, e.g., FIG. 10(b). The excessive supply of thermal energy to the polymer causes the decomposition of a protective group of the polymer and the variation of the formation of the polymer. This is considered to increase the Rmin.
The foregoing suggests that the quality of the film can be optimized by changing the heating value applied to the resist film in accordance with the amount of vilatilized solvent. However, since the ununiformity of the quality of the film can be eliminated by the conventional technique for always baking on constant process conditions regardless of the components of the resist film, the inventor has searched for a new heating method in the PAB.